Molecular and Cell Biology of Cell Cycle Progression Revealed by Mammalian Cells Temperature-Sensitive in DNA Synthesis

Abstract

Studies of the cell cycle arrest of heat-inactivated mutant mammalian cells, which are temperature-sensitive (ts) in DNA synthesis have revealed a number of interesting processes of co-ordinated cell cycle progression. Of special relevance here are the ts A1S9, ts Cl and ts 2 mouse fibroblasts, whose mutant genes encode information for a DNA topoisomerase II enzyme, a DNA chain elongation factor and a function associated with DNA polymerase-α activity, respectively. It has been established that expression of these three defects brings cells into arrest early in S phase, in advance of the hydroxyurea restriction point, early in S phase but after the hydroxyurea execution point, and very late in G₁, at the G₁/S interface, respectively. Temperature inactivation of each unique gene product impacts in a specific way on the synthesis of the major chromosomal proteins, and on the nucleosomal structural organization of the chromatin. In addition, interruption of movement through the cell duplication cycle by heat denaturation of each mutant protein has yielded information concerning the possible involvement of cytoskeletal components in co-ordinating events of cell cycle progression.